Under present conditions, petroleum refineries are finding it necessary to convert increasingly greater proportions of crude to premium fuels such as gasoline and middle distillates such as diesel and jet fuel. Catalytic cracking processes, exemplified by the fluid catalytic cracking (FCC) process and Thermofor catalytic cracking (TCC) process together, account for a substantial fraction of heavy liquids conversion in modern refineries. Both are thermally severe processes which result in a rejection of carbon to coke and to residual fractions; during catalytic cracking high molecular weight liquids disproportionate into relatively hydrogen-rich light liquids and aromatic, hydrogen-deficient heavier distillates and residues.
Catalytic cracking in the absence of hydrogen does not provide significant desulfurization nor is the nitrogen content of the feed selectively rejected with the coke. Both sulfur and nitrogen therefore concentrate appreciably in the heavier cracking products. Cracking therefore produces significant quantities of highly aromatic, hydrogen-deficient middle and heavy distillates that have high sulfur and nitrogen levels. Recycling these liquids to the catalytic cracker is often not an attractive option, because they are refractory and difficult to convert and often will impair conversion of the less refractory fresh feed. Generally, the level of heteroatom contaminants increases with the boiling point of the fraction, as shown in Table 1 below which gives the sulfur and nitrogen contents for two typical FCC product fractions, a light cycle oil and an FCC main column bottoms (proportions and percentages by weight, as in the remainder of this specification unless the contrary is stated).
TABLE 1 ______________________________________ FCC Product Fractions Aromatics, pct. S, pct. N, ppm H, pct. ______________________________________ Light Cycle Oil 80 3.1 650 9.1 Main Column Bottoms 80+ 4.6 1500 6.8 ______________________________________
Present market requirements make refractory product streams such as these particularly difficult to dispose of as commercially valuable products. Formerly, the light and heavy cycle oils could be upgraded and sold as light or heavy fuel oil, such as No. 2 fuel oil or No. 6 fuel oil. Upgrading the light cycle oil was conventionally carried out by a relatively low severity, low pressure catalytic hydro-desulfurization (CHD) unit in which the cycle stock would be admixed with virgin mid-distillates from the same crude blend fed to the catalytic cracker. Further discussion of this technology is provided in the Oil and Gas Journal, May 31, 1982, pp. 87-94.
Currently, however, the refiner is finding a diminished demand for fuel oil. At the same time, the impact of changes in supply and demand for petroleum has resulted in a lowering of the quality of the crudes available to the refiner; this has resulted in the formation of an even greater quantity of refractory cycle stocks. As a result, the refiner is left in the position of producing increased amounts of poor quality cycle streams from the catalytic cracker while having a diminishing market in which to dispose of these streams.
At many petroleum refineries, the light cycle oil (LCO) from the FCC unit is a significant component of the feed to the catalytic hydrodesulfurization (CHD) unit which produces No. 2 fuel oil or diesel fuel. The remaining component is generally virgin kerosene taken directly from the crude distillation unit. The highly aromatic nature of LCO, particularly when the FCC unit is operated in the maximum gasoline mode, increases operational difficulties for the CHD and can result in a product having marginal properties for No. 2 fuel oil or diesel oil, as measured by cetane numbers and sulfur content.
An alternative market for mid-distillate streams is automotive diesel fuel. However, diesel fuel has to meet a minimum cetane number specification of about 45 in order to operate properly in typical automotive diesel engines. Because cetane number correlates closely and inversely with aromatic content, the highly aromatic cycle oils from the cracker typically with aromatic contents of 80% or even higher have cetane numbers as low as 4 or 5. In order to raise the cetane number of these cycle stocks to a satisfactory level by the conventional CHD technology described above, substantial and uneconomic quantities of hydrogen and high pressure processing would be required.
Because of these problems associated with its use as a fuel, recycle of untreated light cycle oil to the FCCU has been proposed as a method for reducing the amount of LCO. Benefits expected from the recycle of LCO include conversion of LCO to gasoline, backout of kerosene from No. 2 fuel oil and diminished use of cetane improvers in diesel fuel. However, in most cases, these advantages are outweighed by disadvantages, which include increased coke make in the FCC unit, diminished quality of the resultant LCO and an increase in heavy cycle oil and gas.
A typical LCO is such a refractory stock and of poor quality relative to a fresh FCC feed that most refineries do not practice recycle of the untreated LCO to any significant extent. One commonly practiced alternative method for upgrading the LCO is to hydrotreat severely prior to recycle to the catalytic cracker or, alternatively, to hydrotreat severely and feed to a high pressure fuels hydrocracker. In both such cases, the object of hydrotreating is to reduce the heteroatom content to low levels while saturating polyaromatics to increase crackability. Although this does enhance the convertibility of these aromatic streams considerably, the economic penalties derived from high hydrogen consumptions and high pressure processing are severe. In addition, in those instances where the production of gasoline is desired, the naphtha may require reforming to recover its aromatic character and meet octane specifications.
Hydrocracking may be used to upgrade the higher-boiling more refractory products derived from catalytic cracking. The catalytic cracker is used to convert the more easily cracked paraffinic gas oils from the distillation unit while the hydrocracker accepts the dealkylated, aromatic cycle oils from the cracker and hydrogenates and converts them to lighter oils. See Petroleum Refining; Second Ed.; Gary, J. H. and Handwerk, G. E.; Marcel Dekker, N.Y. 1984; pp. 138-151; Modern Petroleum Technology, Fourth Ed.; Hobson, G. D., Applied Science Publ. 1973; pp. 309-327. These hydrocracking processes using catalytically cracked feeds either on their own or mixed with virgin feeds have, however, generally been incapable of producing high octane gasoline directly. The reason for this is that they have conventionally been operated at high hydrogen pressures and at relatively high conversion levels so as to maximize the saturation of the aromatics (especially the refractory polynuclear aromatics), removal of heteroatoms in inorganic form and the subsequent conversion of the hydrogenated aromatics to paraffins. While this may produce acceptable diesel fuel (which benefits from the presence of n-paraffins) the octane quality of the gasoline has generally been poor as a consequence of the large quantities of low octane paraffin components. For present day use these gasolines will require extensive reforming with its consequent yield loss in order to conform to market product specifications. To illustrate, U.S. Pat. No. 3,132,090 discloses the use of a two-stage hydrocracking scheme to produce gasoline. However, the octane number of the gasoline using a virgin distillate as charge is reported as 68 (RON+0). An octane of 80 (RON+3) is disclosed for a charge-stock of coker distillate and thermally cracked gas oils. The "high octane" gasolines described in this patent contain 3 ml/gallon of tetraethyl lead (TEL) and are in the range of 70-88 (RON+3). Because TEL adds about 4-6 octane numbers these gasolines have an octane rating on a clear basis (RON+0) in the range of 65-83 (RON+0).
Various low pressure hydrocracking processes have also been described. For example, U.S. Pat. Nos. 3,867,277 and 3,923,640 disclose low pressure hydrocracking processes using various high boiling feedstocks, generally of high (20-40) API gravity. The use of such feeds, coupled with the relatively high levels of conversion in those processes leads to naphthas of low octane rating since the alkyl groups present in the feeds come through into the naphtha together with the relatively straight chain paraffins produced by the ring opening and cracking of the aromatics. These processes have therefore been unsatisfactory for the direct production of high octane gasoline.
Other low pressure hydrocracking processes producing aromatic products have been described in the past but their potential for producing high octane gasoline from low value, refractory cracking oils has not been appreciated. For example, U.S. Pat. No. 4,435,275 describes a method for producing aromatic middle distillates such as home heating oil from high gravity feeds under relatively low conversion conditions but with the objective of producing low-sulfur middle distillates, octane numbers of only about 78 (R+0) are reported.
A notable advance is described in patent application Ser. No. 825,294 to which reference is made for details. It was found that highly aromatic, refractory feeds derived from catalytic cracking could be converted directly to high octane gasoline by hydrocracking at relatively low pressures, typically 600-1000 psig (about 4250-7000 kPa. abs.) and with low conversions, typically below 50 weight percent to 385.degree. F.-(195.degree. C.-) products. (All SI equivalents in this specification are rounded off to a convenient figure so as to permit convenient comparison; all pressures quoted in SI units are absolute pressures). By using a highly aromatic feed which has been substantially dealkylated in the catalytic cracking operation, typically with an API gravity of 5-25, the hydrocracking proceeds with only a limited degree of aromatics saturation so that a large quantity of single-ring alkylaromatics (mainly benzene, toluene, xylenes and trimethyl benzenes) are obtained by ring opening of partial hydrogenation products of bicyclic aromatics. The single ring aromatics are not only in the gasoline boiling range but also possess high octane numbers so that a high octane gasoline is produced directly, suitable for blending into the refinery gasoline pool without prior reforming.